The present invention relates to electrical transformers, and more specifically to high voltage transformers used in equipment for generating electrostatic fields and discharges.
Various types of electrostatic apparatus, such as neon signs, electrostatic filters and copiers, and corona discharge devices, utilize transformers to convert 120 or 240 volts from an alternating electric current source to output levels up to 15 kilovolts at 500 volt-amperes. The high voltage is used to create an electrostatic field within the apparatus. Typically, the transformers for such apparatus comprise a primary coil and a high voltage secondary coil wound on a metal core which provides inductive coupling between the two coils. The coil assembly is placed in a metal container and surrounded by a potting compound. Traditionally, either asphalt or epoxy based compounds are used to pot the coil assembly. The potting compound serves several purposes holding the coil assembly within the container, transferring heat from the coils to the metal container where it dissipates into the environment, electrically insulating the coils from the core and the container, and protecting against moisture penetration. During the potting process, air is often trapped around the coils creating voids between the potting compound and the coil. Failure of these high voltage transformers can result from a corona discharge occurring in the void and consuming the transformer material, including the coil wires. The intensity of the corona in a void is inversely proportional to the size of the void, making even small trapped air bubbles significant to transformer performance. A void can also promote a chemical reaction which also dissolves the insulation in the immediate area.
Despite the voids, some types of high voltage loads place minimal performance demands on these transformers, permitting satisfactory operation with less than optimal designs. For example, a 10 kilovolt transformer used for neon signs is required to produce the full output voltage only briefly during its normal use. Generally, the full voltage will be required only to initiate the gas discharge. Once ignition has been established, the transformer output voltage will drop to one to three kilovolts as a result of the load's tendency to draw down the output voltage.
However, the same transformer will fail within a few hours when it is used in a corona discharge device. This failure occurs for several reasons. The nature of corona discharge loads generally requires 100 percent duty cycle indefinitely. In addition, the operating temperature of the transformer can be elevated for prolonged periods. Due to the relatively capacitive load that a corona discharge device places on the transformer, the output voltage actually rises by about 20 percent. The conventional potted transformers provide less than optimal performance with loads requiring continuous operation at full output voltage.